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I am a beginner with QGIS and I have been working on creating layers with LAStools and shapefiles of the city of Rio de Janeiro.

Both data are provided by the city, so shouldn't contain any error. My two first layers ("cobertura4618" & "Regiao Metropolitana") are aligning perfectly when I project them on the same system EPSG 4618 - SAD 69. But the last layer "output vector file" (created through lastools and projected on the same system) doesn't overlap with the others even if I have projected it on the same system like the others.

when looking at the map extent the polygons are really far from each other even if they should overlapp

the layer which does not overlap with the two other layers

the two layers which overlap without problem

settings

The on-the-fly CRS tranformation is disabled. "Prompt for crs" is selected. I have used always the same method to project the layer on EPSG 4618 - SAD 69 : right click on the layer "save as" > "selected CRS" > ok.

I have read a lot of posts about projection problem but none of them helped to solve my problem.

PolyGeo
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maevad01
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  • Looking at the coordinates in the status bar, it seems that the first two images are in a projected CRS, while the last uses degrees. So EPSG:4618 is only correct for the last one. You might need to assign a SAD69-based projected CRS to the first two. – AndreJ May 14 '18 at 15:11
  • Hi andrej, thank you for your answer! Do you mean that the two first layers "cobertura4618" and "regiao" (corresponds to 3rd Screenshot) are using degrees and the last layer "output vector file" (green polygons, 2nd screenshot) is in a projected CRS? So I have to assign to the two first layers a SAD69-based projected CRS : how do I do this? (sorry I am a beginner) – maevad01 May 14 '18 at 16:50
  • Project your layer to the new CRS by right clicking on the layer name in the Layer Panel, choose save as... then select the new CRS and save it as a new layer. Do not change the CRS in the Layer Properties. – csk May 14 '18 at 18:09
  • Actually, I may have been wrong; this looks like one of the rare cases when you should change the CRS in layer properties. You need to figure out what CRS the output layer was created in. Then change its CRS (in the layer properties). – csk May 14 '18 at 20:07
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    Set the PROJECT CRS to EPSG:3857, load an OpenStreetMap background as a reference, then add your layers one by one. For degree-based CRS, take EPSG:4618, for others you have to try SAD69-based projected CRS. – AndreJ May 15 '18 at 06:09
  • I have just tried your last comment. I have loaded an open street map and set the EPSG 3857. 1) I have Inserted the layer of the state of Rio (with EPSG 4618). When the "on the fly" transformation is on; the state matches with the basemap. When I disable the "on the fly" function, the state of Rio is not matching anymore. 2) The 2nd layer (a small part of Rio) is still problematic. I have tried a SAD69 projected CRS (EPSG: 29193 / UTM zone 23S). When "on the fly projection" is active, the polygons are not appearing at all (even though they are existing). when the on the fly is disactivated, – maevad01 May 16 '18 at 13:36
  • I can see the polygons of this small part of Rio, but it is not well located (somewhere in the ocean). Why does this layer not appear at all when I project it to a SAD69-based projected CRS? Do I miss something? Thank you for your help! – maevad01 May 16 '18 at 13:37
  • Hi! could I please send you the layers so that you have a look at my problem? It's really important for my research to solve it :) thank you! – maevad01 May 18 '18 at 11:18
  • Be aware that to correctly project a layer, one needs to make sure the raw file is assigned with the correct CRS. Otherwise, the starting CRS is wrong, so it will be the new one (the transformed one). Hence, make sure the CRS assigned to each of your layers is correctly defined, then, transform all of them to same CRS. See here: https://gis.stackexchange.com/questions/20572/layers-with-same-coordinate-system-should-align-overlap-in-arcmap-but-do-not – Andre Silva Jan 17 '19 at 14:02

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